Semantic memory refers to knowledge about objects, facts, concepts, and words and their meanings. Interest in the neural bases of semantic memory comes from observations of patients with striking impairments in semantic memory despite otherwise intact cognitive functioning. Such impairments are observed most commonly in degenerative diseases: semantic memory impairments are one of the earliest and most prominent features of Alzheimer's disease, and loss of semantic knowledge is the defining feature of a syndrome known as "semantic dementia", associated with Pick's disease. Impairments of semantic memory have also been observed in patients with herpes simplex encephalitis, posterior cerebral artery infarctions, hypoxic-ischemic injury, and tumor resection. These observations have led to speculation about the neural bases of semantic memory, aided by the development of modern neuroimaging techniques for studying normal cognition. These two methods (lesion studies and imaging studies) have identified two regions that may be specialized for semantic memory: left temporal cortex and left prefrontal cortex. Based on an analysis of the literature and some preliminary findings, we hypothesize that prefrontal and temporal cortex play quite distinct roles in semantic memory; and that prefrontal cortex is necessary for the selection of competing information, and not for semantic retrieval per se. The proposed project has four major goals: (1) to demonstrate a dissociation between the roles of prefrontal cortex and temporal cortex in semantic memory; (2) to describe in more precise terms, both cognitively and neurally, the nature of semantic repetition effects in temporal cortex; (3) to develop further the hypothesis that prefrontal cortex subserves the selection of competing information and to distinguish this hypothesis from competing alternatives; (4) to provide converging evidence as to the neural bases of semantic memory with complementary imaging and lesion methodologies.